Overlapping Expression and Redundant Activation of Mesenchymal Fibroblast Growth Factor (FGF) Receptors by Alternatively Spliced FGF-8 Ligands

Abstract

FGF-8 is a member of the family of fibroblast growth factors and is expressed during vertebrate embryo development. Eight potential FGF-8 isoforms are generated by alternative splicing in mice, several of which are expressed during embryogenesis in epithelial locations. The significance of the multiple isoforms is currently unknown. In this report, we investigate the expression patterns and the specificity of the FGF-8 isoforms for known fibroblast growth factor (FGF) receptors. RNAs for seven of the eight potential isoforms are present at multiple sites of embryonic Fgf8 expression. None of the FGF-8 isoforms exhibited activity when assayed with BaF3 cells expressing the "b" splice forms of FGF receptors 1-3, which are mostly expressed in epithelial tissues. Mesenchymally expressed "c" splice forms of FGF receptors 2 and 3 and FGF receptor 4 were activated by several FGF-8 isoforms. These findings are consistent with the hypothesis that the multiple FGF-8 isoforms are functionally redundant and function to signal in paracrine (epithelial to mesenchymal) contexts.The mammalian fibroblast growth factor (FGF) 1 family currently consists of structurally related polypeptides encoded by 10 genes (FGF-1-10) (reviewed in Ref. 1;. Four distinct genes code for high affinity transmembrane receptor tyrosine kinases (FGFR1-4) that bind FGF ligands and display varying patterns of expression (reviewed in Ref. 5). Alternative mRNA splicing generates isoforms of receptors 1-3 that exhibit unique ligand binding characteristics (6 -9). FGF receptor activation involves ligand binding and receptor dimerization, followed by transphosphorylation of the receptor and transduction of the signal into a biological response (5). FGF signal transduction has been implicated in development, wound healing, angiogenesis, and tumorigenesis (reviewed in Ref. 1). Germ line mutations of FGFs and FGFRs in mice (10 -15) and humans (16 -21) demonstrate the importance of FGF signaling in the process of development.FGF-8 was first identified as an androgen-induced growth factor secreted by the mouse mammary tumor cell line SC-3 (2). Subsequently, Fgf8 was identified as a Wnt1-cooperating protooncogene in murine mammary tumorigenesis (22). Fgf8 expression has been detected during murine and chicken embryogenesis in regions of outgrowth and patterning, including the primitive epiblast, the apical ectodermal ridge of the limb bud, the primitive streak, the tail bud, the facial primordia, and the midbrain-hindbrain junction (22-29). The murine and human genes encoding FGF-8 have been localized to mouse chromosome 19 and human chromosome 10q24 -26 (25, 26, 30, 31). The murine Fgf8 gene is unusual in the FGF family in that there are four exons (exons 1A-1D) equivalent to the usual exon 1 in other FGF genes (22,25,32). Alternative splicing of the four alternatively spliced exons results in potentially eight protein isoforms that differ at their amino termini and share a common carboxyl terminus encoded by exons 1D, 2, and 3 (22,25,32). Human FGF8 is similar ...

Increased expression of fibroblast growth factor 8 in human breast cancer

“…Studies have demonstrated that it acts through FGFR2c, FGFR3c, FGFR4 and FGFR1 (MacArthur et al, 1995c;Ornitz et al, 1996;Blunt et al, 1997;Kouhara et al, 1994) and we have shown, by PCR and immunohistochemistry, that FGFR4 and FGFR1 are present in breast cancers allowing a potential autocrine loop to develop. If there are a proportion of breast cancers that are driven by such a loop then interrupting this cycle could lead to the development of novel therapies.…”

FGF8 isoform b expression in human prostate cancer

“…Four receptors have been detailed (designated FGFR 1 -4) and multiple splice variants of these receptors have been described (Ornitz et al, 1996). FGF8 is known to activate FGFR2, 3 and 4, as well as FGFR1 if applied in high concentrations (Ornitz et al, 1996;Blunt et al, 1997). In addition, developmental studies have shown that it is the c splice forms of FGFRs that are the preferred targets of FGF8 and these are expressed predominantly in the mesenchymal tissue (MacArthur et al, 1995b).…”

“…In this context, FGF8 may well adopt both a paracrine and an autocrine role. The FGF8 isoforms themselves are known to have relatively specific binding characteristics (Blunt et al, 1997). Human FGF8a, for example, does not activate any of the FGFRs, while the FGF8b and e isoforms are known to interact with FGFR3IIIc and FGFR4 (Blunt et al, 1997).…”

FGF8b oncogene mediates proliferation and invasion of Epstein–Barr virus-associated nasopharyngeal carcinoma cells: implication for viral-mediated FGF8b upregulation

“…Investigation on the involvement of FGF8b in hormone-related cancers was indeed based on the initial discovery of FGF8 (androgen-induced growth factor) in an androgen-dependent mouse mammary carcinoma cell line (Tanaka et al, 1992) and detection of FGF8 overexpression in human breast and prostate cancers (Tanaka et al, 1995;Leung et al, 1996;Gnanapragasam et al, 2002). Further genomic and functional characterization of the FGF8 family demonstrated that the FGF8b isoform (one of the four human FGF8 isoforms: FGF8a, b, e and f (Gemel et al, 1996)) possesses the greatest mitogenic and, more importantly, the greatest transforming activity than the other isoforms, both in vitro and in vivo (MacArthur et al, 1995a(MacArthur et al, , 1995bBlunt et al, 1997;Song et al, 2000;Olsen et al, 2006). It is believed that FGF8b is one of the most important FGF8 isoforms in human carcinogenesis Gnanapragasam et al, 2003), although the exact physiological function of each isoform is yet-to-be fully revealed.…”